1. Field of the Inventin
The present invention relates to an engine-control apparatus.
2. Related Art
In order to reduce sounds that arise upon discharge of the burnt exhaust gases from the engine, a muffler or a silencer is connected to an exhaust passageway. The sounds that generates from the engine may be classified roughly into two types: one type being a sound which is generated by a flow of the exhaust gases to a remarkable extent at the time of high output of the engine where the pressure of the exhaust gases, namely, the exhause pressure, is increased and which gets lower as the exhaust pressure is increased and the other type being a sound which is generated to a remarkable extent at the time of low output of the engine and which gets lower as the exhaust pressure is decreased. It is thus to be noted that either of the two types of the sounds is enlarged in such a state of running where the other type is reduced. Accordingly, the sound caused by the exhaust gases cannot be reduced efficiently using one muffler or silencer because one of the two types of the sounds is generated to a greater extent in a situation whee the other is lowered. Recently, there is an increasing trend that a plurality of mufflers or silencers are mounted on the exhaust passageway so as to operate a shift valve to open or close at least a portion of the sound deadening passageway.
For example, Japanese Patent Publication (kokai) No. 74,325/1984 discloses a system in which plural mufflers or silencers different in characteristics from each other are connected to an exhaust passageway in such a manner that a flow of the exhaust gases to at least a portion of the plural silencers is switched by a shift valve. More specifically, the exhaust passageway is branched into a first branch exhaust passageway and a second branch exhaust passageway, both serving as sound deadening passages, the first branch exhaust passageway being mounted with a first silencer designed to be used exclusively at the time of low rotational speeds of the engine while the second branch exhaust passageway being mounted with a second silencer designed to be used exclusively at the time of high rotational speeds thereof. The shift valve mounted on the exhaust passageway is furthermore designed to switch the flow of the exhaust gases in accordance with engine load in such a manner that the exhaust gases flow through the first branch exhaust passageway at the time of the low engine load while they flow through the second branch exhaust passageway at the time of the high engine load.
Furthermore, Japanese Utility Model Publication (kokai) No. 58,611/1986 discloses an exhaust system in which one silencer or muffler is provided with two exhaust outlet tubes different in diameter from each other, with a shift valve mounted on the large-diameter exhaust outlet tube. The shift valve is operated to close the large-diameter exhaust outlet tube at the time of low rotational speeds of the engine where an amount of the flow of the exhaust gases is small, on the one hand, thereby flowing the exhaust gases merely through the small-diameter exhaust outlet tube. It is operated to open the large-diameter exhaust outlet tube at the time of high rotational speeds thereof where an amount of the flow of the exhaust gases is large, on the other hand, thus capable of flowing the exhaust gases through both the large-diameter and small-diameter exhaust outlet tubes.
It should be noted, however, that the pressure of the exhaust gases change to a considerably great extent as the sound deadening passages are switched by the shift valve leading to a great change in torque generated by the engine and causing a torque shock. As a change of torque occurs against the driven wheels, the change of torque may be perceived as a considerably large degree of shock against the vehicle body itself.